This invention concerns an electrolyte for electroless deposition of metal layers with internal compressive stresses containing a metal base salt, a reducing agent, a complexing agent, an accelerator and a stabilizer, where preferably nickel, copper, silver or gold, especially preferably nickel, is used as metal.
Besides electrolytic methods for coating workpieces with a metal layer, the so-called noncurrent or currentless coating methods (electroless plating) have long been known. Electroless or chemical metalizing is understood to mean chemical surface treatment of nearly all metals and many nonconductors. This treatment differs in its chemical, physical and mechanical characteristics considerably from electrolytically deposited metal coatings. For example, it is advantageous that the chemical metal coating is produced uniformly in the deepest drillings and passages and, moreover, a layer thickness that is nearly constant and that follows contours more precisely is produced. This method is employed especially frequently for coating nonconducting substrates, for example plastic parts, in order to make them conductive, for example by means of a metallic surface, and/or to give them an attractive appearance. In the same way the material properties of the thus treated substrates can be improved by such methods. For instance, in each case according to the process, the corrosion resistance, hardness and/or wear resistance of the material can be improved.
Electroless coating with metals is based on an autocatalytic process, so that it is also called autocatalytic coating or plating. In order to reduce metal ions contained in the deposition bath (electrolyte) to elemental metal in such coating processes, an appropriate reducing agent that is itself oxidized during the reaction must be added to the electrolyte. Moreover, other components, for example phosphorus and/or additional metals like copper, etc. are often also incorporated into the coating.
Thus, metal coatings with a relatively high phosphorus content are produced in the case of an electroless metal bath through the use of hypophosphite as reducing agent. The corresponding reaction equation for this is as follows:MSO4+6NaH2PO2→M+2H2+2P+4NaH2PO3+Na2SO4 
Since the amount of phosphorus has a considerable effect on the properties of the coating, for example hardness and corrosion resistance, it is incorporated in a controlled way, in each case according to the purpose of the coated object. For example, a phosphorus fraction of ≧10 wt % is desired in the case of nonmagnetic coatings with maximum hardness. Moreover, such electroless deposited metal-phosphorus coatings have higher hardness and better wear resistance than electrolytically deposited coatings.
Hypophosphite-containing baths for electroless deposition of metals, however, tend to become unstable during the deposition, since the concentration of the metal and hypophosphite ions continuously decreases with progressive metal plating, while the concentration of orthophosphite ions continuously increases and the counterions of the metal and hypophosphite ions, in the form of sodium sulfate, for example, increase in concentration. The electrolyte in this way becomes “spent.”
The lifespan of such electroless baths is thus limited, since the electrolyte can be used only for a certain number of coating runs with uniform coating results. The age of a bath is usually given in terms of metal turnover (MTO), where 1 MTO is equal to the amount of metal deposited from the bath. This corresponds to the originally introduced concentration of metal ions in the bath, in each case with respect to the total volume of the bath. In the case of the methods that are currently known in the prior art the degradation products in the electrolyte reach a concentration after 5 to 10 MTO that is so high that a high deposition rate as well as a uniformly high quality of the deposited metal can no longer be guaranteed. The electrolyte must then either be replaced or regenerated by means of appropriate agents.
However, the necessary disposal of the spent baths and the necessary recharging of fresh baths leads, disadvantageously, to high costs and considerable environmental stress.
The regeneration of an electrolyte for nickel deposition means at least the removal of the orthophosphite ions that have been formed as reaction products and optionally the addition of metal or hypophosphite ions. In the known methods the troublesome components are separated from the bath, for example, by means of adsorption on ion exchange resins or by electrodialytic processes. Such processes do enable a considerably longer bath life time, but for the most part they involve very high operating costs because of the complex apparatus, etc.
Another less costly method of regeneration of baths for electroless deposition of metals is the in situ deposition and separation of undesired ions in the form of sparingly soluble compounds and the subsequent makeup of ions that are necessary and that are consumed in the course of the bath service life. However, for the most part only rare metals, which are very expensive, are possibilities as precipitation agents. In addition, the components of these additions that remain dissolved in the bath have an adverse affect on the quality of the metal coating.
In addition, there are already known methods in which troublesome deposits of metal orthophosphite can be prevented through the addition of complexing agents, and in this way the stability of the baths can be considerably improved through the targeted reduction of the concentration of dissolved free nickel ions. In the past many different types of bath additives have been proposed, but they all had the disadvantage that uniform, pore-free and firmly adhering deposition of metal-phosphorus coatings is not possible over a long period of time from such baths at an economically acceptable deposition rate of 7-10 μm/h and with internal compressive stresses at a phosphorus content of the coating of >10%. Usually the lifetime of such baths is 7 to a maximum of 10 MTO, where no S2−-containing accelerators are used.